LED-driver circuits can be
dimmed by applying a variable
duty cycle (pulse-width
modulation, or PWM) to the
LED. PWM exploits LED behavior:
At higher current levels, the
LED’s light output is higher for a
given level of power dissipation
(temperature). Thus, applying
PWM current to the LED yields
an average power comparable to
that of dc control, but with higher
operating current and greater
light output.
Even if the available control
signal is a dc level, you can
implement PWM control with
a simple circuit that provides
predictable behavior and good
linearity. Consisting of a dual
comparator and a few external
components, the circuit produces
a 500-Hz PWM signal
with 2% nonlinearity and a duty
cycle adjustable from 0% to 100%, using a 0- to 5-V control
signal (Fig. 1).
The comparator’s “B” side is configured as an oscillator running
at about 500 Hz. It produces a triangular waveform at the
positive input, with an amplitude about one-tenth the supply
voltage. The “A” side compares the triangle waveform with the
dc control signal and generates a PWM signal at OUTA. The
PWM generator and dc control-signal generator should run off
the same +5-V supply.
You can adjust the oscillation frequency (f) by changing the
value of capacitor C in the equation: f = 6345/RC, where R is in
kilohms, C is in microfarads, and f is in kilohertz. To get 500 Hz
in the example circuit, C = 0.47 µF and R = 27 kΩ.
Figure 2 shows the circuit response as PWM duty cycle versus
dc control-signal level.
